Several proteins that have been implicated prominently in long-term potentiation (LTP), a cellular model of learning, are housed in dense-core granules (DCGs) in neurons of the hippocampus and are secreted from these neurons by regulated exocytosis of DCGs. DCGs in hippocampal neurons, and neuronal DCGs generally, have not been well studied, despite their importance as organelles that store, transport, and mediate secretion of critical proteins. Moreover, our recent data highlight significant differences between DCGs in hippocampal neurons and their better-studied neuroendocrine counterparts. In light of this, we propose experiments directed at determining mechanisms underlying two pivotal processes that bear on LTP: the release and retrieval of DCG cargo proteins in hippocampal neurons. Our studies will involve direct visualization of protein release from individual DCGs localized at post-synaptic and other subcellular sites. The major goals of Specific Aim 1 are to determine exocytotic mechanisms used by DCGs in hippocampal neurons, and associated protein release kinetics, induced by a spectrum of electrical stimulation paradigms, including those linked to LTP. The major goals of Specific Aim 2 are to determine mechanisms underlying retrieval of DCG proteins induced by a spectrum of stimulation paradigms, and to determine if a previously fused DCG that is retrieved relatively intact and retains protein will fuse again and release retained protein. Our results will elucidate release and retrieval processes that are implicated in the regulation of DCG protein release, in the facilitation of DCG protein reuse, and in the modulation of release and retrieval of DCG proteins to meet the demands of different inputs. Moreover, our results will provide fundamental insight into cellular processes that may underlie LTP and will reveal behavior of a DCG protein that has been implicated in physiological and pathological functions in the nervous system, including learning, memory, and neurotoxicity associated with Alzheimer's disease. The studies in this proposal focus on elucidating mechanisms underlying the release and retrieval of dense-core granule proteins in hippocampal neurons. The results will provide fundamental insight into cellular processes that may underlie long-term potentiation, a cellular model of learning, and will elucidate the behavior of tissue plasminogen activator, a dense-core granule protein that has been implicated in physiological and pathological functions in the nervous system, including learning, memory, and neurotoxicity associated with Alzheimer's disease. [unreadable] [unreadable]